Method for production of acrylic acid

ABSTRACT

A method is disclosed which produces acrylic acid in a high yield as maintaining the conditions for purifying acrylic acid in constant ranges and preventing the acrylic, acid from polymerization. By using single a single reactor, propylene concentration adjusting in the range of 7-15 vol. % and water concentration adjusting in the range of 0-10 vol. % are introduced thereinto thereby obtaining an acrylic acid-containing gas. Then the gas is introduced to an acrylic acid absorption column to adjust water concentration in the range of 1-45 wt. %, thereby preventing from polymerization.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] This invention relates to a method for producing acrylic acidcomprising introducing propylene as a raw material in high concentrationinto a reactor which has a first reaction zone and a second reactionzone formed by dividing with at least one perforated tube plate, andimproving water concentration in an acrylic acid-containing solution inan acrylic acid absorption column to a level in the range of 1-45 wt. %,thereby preventing the solution from polymerization at the subsequentsteps in the process of the production.

[0003] 2. Description of the Related Art:

[0004] Acrylic acid is used in coating material, fiber processing,leather processing, and building material besides being used for acrylicfiber copolymer or as an emulsion for adhesive agents. The demand forthis compound is now increasing. Thus, it is generally produced by thereaction of catalytic gas phase oxidation of propylene, for example,with a view to realizing the principle of mass production using aninexpensive raw material. The raw material gas is generally supplied ata concentration in the range of about 4-7 vol. % because the reaction ofcatalytic gas phase oxidation has the possibility of belonging to therange of explosion, depending on the proportion of molecular oxygen tobe used in the reaction of oxidation. Since the raw material gas ispreferred to be used at a high concentration for the purpose ofimproving the efficiency of production, various modifications have beenproposed for the supply of the raw material gas at a high concentration.

[0005] The official gazette of JP-A-2000-103761, for example, disclosesa method for producing acrylic acid from propylene by supplying areaction composition comprising more than 7 vol. % of propylene,molecular oxygen, steam, and the balance of inert gas to a reactorfurnished with two reaction zones packed with a catalyst and having amultiplicity of reaction tubes duly distributed therein. When the mixedproduct gas obtained by the reaction of catalytic gas phase oxidation isintroduced into an absorber to obtain an aqueous acrylic acid solution,the exhaust gas of the absorber contains unreacted propylene, unreactedacrolein, and acrylic acid as residues. When this exhaust gas is put tocyclic use, the coefficient of absorption is depressed and the yield ofacrylic acid is degraded because the volume of the inert gas flowingthrough the absorber is increased thereby being generated a load in thetop of the absorber. Conversely, when the exhaust gas is not put tocyclic use, the water content in the gas flowing through the absorber isvaried because the selectivity, the ratio of addition, and the catalyticactivity in the reaction of oxidation are affected by the water contentowing to the use of steam as a diluting gas. The method disclosed in theofficial gazette mentioned above contemplates reducing the load exertedon the absorber by using propylene of higher concentration than usualheretofore in the system of a single reactor and decreasing the exhaustgas of the absorber held as necessary for the sake of dilution.

[0006] Incidentally, after the absorption of the acrylic acid-containinggas, such purification steps as the dehydration of the acrylicacid-containing solution and the separation of low boiling substance andhigh boiling substance contained therein are carried out.

[0007] Acrylic acid, however, is an easily polymerizing compound. Whenthe raw material gas of high concentration is used with a view toheightening the productivity of the method, this method is liable togenerate an acrylic acid polymer at the step for absorbing acrylic acidand the subsequent step for purification of the absorbed acrylic acid.Since various columns for purificating continue production of acrylicacid while preventing occurrence of polymerization by adjusting suchfactors as the pressure of distillation, temperature, and amount offeed, the control of these factors is not easy because changes in theseconditions affect other conditions. When the concentration of acrylicacid is depressed by a variation in the composition of the bottom liquidof an absorption column, for example, the maintenance of distillationconditions in a high acrylic acid concentration entails generation of anacrylic acid polymer in a distilling column. Though it is not to theextent of requiring forced suspension of the operation due to blockage,but possibly results in degrading the quality of acrylic acid produceddue to the inclusion of the polymer and consequently depressing theyield of the product.

[0008] The purification step of acrylic acid is accomplished more oftenthan not by adopting distilling columns called a dehydrating column, alight ends cut column, and a heavy ends cut column, concatenating theplurality of distilling columns endowed with different functions, andoperating them jointly thereby performing a series of purification.Thus, the control of this purification step is rendered furtherdifficult by the fact that a variation in the composition of the bottomliquid of any one of such distilling columns necessitates a due changein the purification conditions of the subsequent steps.

[0009] The solution, the exhaust gas, and the like which are dischargedfrom the step for producing acrylic acid at times contain the rawmaterial compound, the product, and other useful compounds. The cyclicuse of such discharged substances in the process of production,therefore, can improve the yield of production. The composition of theexhaust gas, for example, is varied as with the distillation conditionsand the generation of the polymer proceeds like a chain reaction. In themethod for the production of acrylic acid particularly included a stepfor recycling the exhaust gas, therefore, it is extremely difficult tocontrol the distillation conditions constant.

SUMMARY OF THE INVENTION

[0010] The present inventor has found that when in a process for theproduction of acrylic acid from propylene as a raw material theconditions in a reactor from the reaction of catalytic gas phaseoxidation to the absorption of acrylic acid in an acrylic acidabsorption column are restricted within specific ranges, the acrylicacid can be produced with high productivity from propylene of highconcentration in the reactor and the operation of the process forpurification can be simplified. This invention has been perfected on thebasis of this knowledge.

[0011] Particularly by limiting the water content in the bottom liquidof the absorption column in the range of 1-45 wt. %, the occurrence of apolymer at the subsequent steps of the process can be effectivelyprevented. This control of the water content can be attained byadjusting the amount of an absorbent. By effecting this control in amore specific section of the range of 1-45 wt. %, it is made possible tonarrow the width of control of the distillation conditions at thesubsequent steps, restrain the fluctuations of loss of acrylic acid inthe absorption column and a waste water generated from the processrespectively to the minimum, and secure the stability of operation atthe subsequent steps including the equipment for the treatment of thewaste water.

[0012] It has been found that the adjustment of the concentration ofabsorption particularly by the variation of the amount of the absorbentin the absorption column results in enabling acrylic acid to be absorbedat a high concentration without changing the amount of the waterdischarged from the top of the absorption column and this adjustment,particularly when the exhaust gas of the absorption column is put tocyclic use, is at an advantage in stabilizing the conditions ofproduction.

[0013] According to this invention, in the process for producing acrylicacid from propylene as the raw material having a step of the reaction ofcatalytic gas phase oxidation using a reactor having a first reactionzone and a second reaction zone thereof formed by dividing reactiontubes with at least one perforated tube plate and a step of the step forabsorbing acrylic acid, by restricting in specific ranges the conditionsin the operation of production, it is made possible to produce acrylicacid with high productivity from propylene gas of high concentration inthe reactor and facilitate the operation in the process of purificationas well.

[0014] Particularly the treatments at the subsequent steps can berendered simple and easy by adjusting the water content of the bottomliquid of the absorption column in the range of 1-45 wt. %.

[0015] When the amount of the absorbent to be supplied in the absorptioncolumn is varied, the acrylic acid can be discharged at a highconcentration without changing the amount of the water absorbed from thetop of the absorption column. This variation brings an excellent effectof stabilizing the conditions of production particularly when theexhaust gas of the absorption column is put to cyclic use.

BRIEF DESCRIPTION OF THE DRAWING

[0016]FIG. 1 is a flow sheet illustrating part of a process forproducing acrylic acid by the use of a single reactor having a firstreaction zone and a second reaction zone thereof formed by dividingreaction tubes with at least one perforated tube plate.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0017] The first aspect of this invention is a method for the productionof acrylic acid comprising a step of introducing a mixed gas containingpropylene and molecular oxygen into a first reaction zone packed with acomplex oxide catalyst having molybdenum and bismuth as essentialcomponents and oxidizing propylene and obtaining an acrolein-containinggas, a step of introducing said acrolein-containing gas into a secondreaction zone packed with a complex oxide catalyst having molybdenum andvanadium as essential components and obtaining an acrylicacid-containing gas, and a step of introducing said acrylicacid-containing gas into an acrylic acid absorption column and causingit to contact an absorbent thereby obtaining an acrylic acid-containingsolution which comprises the steps of

[0018] (a) said first reaction zone and said second reaction zone beingformed by dividing reaction tubes with at least one perforated tubeplate,

[0019] (b) said mixed gas for introduction into said first reaction zonehaving a propylene concentration in the range of 7-15 vol. % and a waterconcentration in the range of 0-10 vol. %, and

[0020] (c) said acrylic acid-containing solution absorbed in saidacrylic acid absorption column having a water concentration in the rangeof 1-45 wt. %.

[0021] For the sake of heightening the yield of production, it iseffective to increase the concentration of propylene and necessary aswell to increase the concentration of oxygen. Since the absorption ofthe heat of reaction consequently generated is not sufficient and thecontrol of the reaction is difficult, however, there are times when thereaction of catalytic gas phase oxidation will not be accomplished in astable manner on account of the relation thereof with the explosiverange. In this case, the explosive range mentioned above can be avoidedby predetermining an explosive range conforming to the conditions of anassumed operation and, when the operation is found to fall in thepredetermined explosive range, introducing into the reaction system sucha diluting gas as propane which can narrow this range. It has been foundthat by simultaneously adjusting the water concentration of the acrylicacid-containing solution which has absorbed the produced acrylic acid toa level in the range of 1-45 wt. %, it is made possible to prevent thegeneration of a polymer in the absorption column and at the subsequentsteps and mitigate the operation, of purification at the subsequentsteps. The term “purification” as used in this invention embracesdistillation, stripping, crystallization, extraction, absorption,dephlegmation, etc. The term “distillation” means a method whichconsists in heating a solution to the boiling point thereof andseparating from the solution a volatile component contained therein, theterm “strippings” means a method which consists in supplying a strippinggas into a liquid and effecting transfer of an object substance in theliquid phase into a gas phase, the term “crystallization” means a methodwhich consists in separating an object substance in the form ofcrystals, the term “extraction” means a method which consists inseparating an object substance by dissolving it in a solvent, the term“absorption” means a method which consists in separating an objectsubstance in a gas phase or a liquid phase by causing it to contact aliquid or a solid, and the term “dephlegmation” means a method whichconsists in separating an object substance by causing part of a gas orvapor to condense. The term “low boiling substance” as used in thisinvention refers to a substance which has a lower boiling point thanacrylic acid in the standard condition and the term “high boilingsubstance” refers to a substance which has a higher boiling point thanacrylic acid in the standard condition. Now, this invention will bedescribed in detail below.

[0022] This invention obtains an acrylic acid-containing gas byintroducing propylene into a first reaction zone packed with a complexoxide catalyst having molybdenum and bismuth as essential componentthereby oxidizing propylene and forming an acrolein-containing gas andintroducing the acrolein-containing gas into a second reaction zonepacked with a complex oxide catalyst having molybdenum and vanadium asessential components.

[0023] The reactor does not need to be particularly restricted but isonly required to be capable of performing a reaction of catalytic gasphase oxidation. A shell-and-tube type reactor can be advantageouslyused because it excels in the efficiency of reaction. The material,size, and quantity of reaction tubes and the distribution of such tubesmay be arbitrarily selected. Thus, any of the known test tubes may beused. This invention is characterized by using a reactor which has afirst reaction zone and a second reaction zone thereof formed bydividing reaction tubes with at least one perforated tube plate(hereinafter referred to as “single reactor”). The first reaction meansa step of oxidizing propylene and converting it mainly into acrolein andthe second reaction means a step of oxidizing acrolein into acrylicacid. As for the oxidizing catalyst destined to fill the reaction zone,the first reaction zone is packed with the complex oxide catalyst (i)having molybdenum and bismuth as essential components for obtainingacrolein by oxidizing propylene and the second reaction zone is packedwith the complex oxide catalyst (ii) having molybdenum and vanadium asessential components for obtaining acrylic acid by oxidizing acrolein.

[0024] One example of the embodiment of this invention using one reactorhaving the first reaction zone and the second reaction zone thereofpartitioned with a perforated tube plate will be explained below withreference to FIG. 1. In FIG. 1, 1 denotes propylene, 2 steam, 3 air, 10a reactor, 11 a reaction tube, 12 a complex oxide catalyst (i), 13 acomplex oxide catalyst (ii), 15 a perforated tube plate, 20 an acrylicacid-containing gas, 23 a heat exchanger, 30 an absorption column, 31 apacked bed, 32 a distributor, 33 an absorbent, 34 a cooler, 35 anacrylic acid-containing solution, 36 exhaust gas, 40 a dehydratingcolumn, 41 a distilled vapor, 42 a solvent phase, 43 a water phase, 50 asubsequent step, 60 waste gas, 71 a heat exchanger, and 72 a flowcontroller.

[0025] First, the propylene 1 as the raw material gas is suppliedtogether with the steam 2 and the air 3 containing molecular oxygen tothe reactor 10. In the reactor 10, the reaction tube 11 is packed withthe complex oxide catalyst (i) 12 and the complex oxide catalyst (ii) 13as oxidizing catalysts and the catalyst (i) oxides propylene and obtainsacrolein and then the catalyst (ii) oxides acrolein and obtains theacrylic acid-containing gas 20. Subsequently, the acrylicacid-containing gas 20 is introduced into the acrylic acid absorptioncolumn 30 and is absorbed by the absorbent 33. The bottom liquid of thecolumn as the acrylic acid-containing solution 35 is partly circulatedto the acrylic acid absorption column 30 as kept cooled with the cooler34, and the rest is introduced into the dehydrating column 40 andsubjected therein to a dehydrating treatment. The distilled vapor 41from the dehydrating column 40 is condensed by the condenser disposed atthe top of the dehydrating column 40 and then the resultant condensateis separated into the solvent phase 42 and the water phase 43. Thesolvent phase 42 is circulated to the dehydrating column 40 and thewater phase 43 may be reused as the absorbent 33. The bottom liquid ofthe dehydrating column 40 is supplied to the subsequent step 50 andfurther subjected to purification for the separation of the low boilingsubstance and the high boiling substance. The exhaust gas 36 from theabsorption column 30 may be wholly disposed of as the waste gas 60.Alternatively, part of the exhaust gas 36 may be circulated by theblower 4 to the reactor 10 as a recycle gas and only the remainderthereof may be disposed of as the waste gas 60.

[0026] In this invention, the complex oxide catalyst (i) which is usedin the first reaction zone is only required to contain molybdenum andbismuth. A catalyst which is represented by the general formula,Mo_(a)—Bi_(b)—Fe_(c)-A_(d)-B_(e)-C_(f)-D_(g)-O_(x)(wherein Mo, Bi, andFe denote molybdenum, bismuth, and iron respectively, A denotes at leastone element selected from the group consisting of nickel and cobalt, Bdenotes at least one element selected from the group consisting ofalkali metals and thallium, C denotes at least one element selected fromthe group consisting of phosphorus, niobium, manganese, cerium,tellurium, tungsten, antimony and lead, D denotes at least one elementselected from the group consisting of silicon, aluminum, zirconium andtitanium, O denotes oxygen, and a, b, c, d, e, f, g, and x denote atomicratios respectively of Mo, Bi, Fe, A, B, C, D, and O such that b=0.1-10,c=0.1-10, d=2-20, e=0.001-5, f=0-5, and g=0-30 are satisfied when a=12is assumed, and x represents the value which is fixed by the states ofoxidation of the relevant elements), may be cited as a preferredexample.

[0027] Then, the complex oxide catalyst (ii) is only required to containmolybdenum and vanadium. A catalyst which is represented by the generalformula, Mo_(a)—V_(b)—W_(c)—Cu_(d)-A_(e)-B_(f)-C_(g)-O_(x) (wherein Modenotes molybdenum, V denotes vanadium, W denotes tungsten, Cu denotescopper, A denotes at least one element selected from the groupconsisting of antimony, bismuth, tin, niobium, cobalt, iron, nickel andchromium, B denotes at least one element selected from the groupconsisting of alkalimetals and alkaline earth metals, C denotes at leastone element selected from the group consisting of silicon, aluminum,zirconium and titanium, O denotes oxygen, and a, b, c, d, e, f, g, and xdenote the atomic ratios respectively of Mo, V, W, Cu, A, B, C, and Osuch that b=2-14, c=0-12, d=0.1-5, e=0-5, f=0-5, and g=0-20 when a=12 isassumed, and x represents the value which is fixed by the state ofoxidation of the relevant elements), may be cited as a preferredexample.

[0028] The methods for preparing the catalysts to be used in thisinvention and the methods for mixing the component materials and moldingthe resultant mixtures do not need to be particularly restricted. Theymay be selected from the methods and the raw materials which are inpopular use. The shapes of the catalysts contemplated by this inventiondo not need to be particularly restricted. The catalysts may be formedas spheres, circular columns, and cylinders, for example. The methods ofmolding may be stationary molding, extrusion molding, pellet molding,etc. Further, the catalysts which have catalytic substances deposited onrefractory carriers may be used.

[0029] In this invention, the propylene concentration in the mixed gaswhich is introduced into the first reaction zone is preferably in therange of 7-15 vol. %, more preferably in the range of 8-12 vol. %, andparticularly preferably in the range of 8-10 vol. %. If the propyleneconcentration falls short of 7 vol., the shortage will possibly resultin depressing the efficiency of production and lowering the absorbedacrylic acid concentration. Conversely, if the propylene concentrationexceeds 15 vol. %, the excess will possibly result in causing thereactor to enter the explosive range and aggravating the hazardousnessof the operation.

[0030] The water concentration in the mixed gas to be supplied to thefirst reaction zone is preferably in the range of 0-10 vol. %, morepreferably in the range of 0-7 vol. %, and particularly preferably inthe range of 0-6 vol. %. The water to be used is wholly transferred tothe absorption column. If the water concentration of the mixed gasexceeds 10 vol. %, therefore, the excess will be at a disadvantage inincreasing the water concentration of the bottom liquid of theabsorption column. It has been elucidated that an attempt to keep thewater concentration of the bottom liquid of the absorption column lowresults in depressing the efficiency of absorption in the absorptioncolumn and heightening the polymerizability of the liquid in theabsorption column and the piping of the absorption column. The term“efficiency of absorption (%)” as used herein refers to the magnituderepresented as 100×{(AAin-AAout)/AAin}, wherein AAin denotes the amountof acrylic acid introduced into the absorption column and AAout denotesthe amount of acrylic acid discharged from the top of the absorptioncolumn. According to this invention, the efficiency of absorption ofacrylic acid in the absorption column can be made to exceed 95%,preferably 96%, by adjusting the water concentration to a level of notmore than 10 vol. %.

[0031] The mixed gas mentioned above is required to include thereinmolecular oxygen. The volume ratio of propylene: molecular oxygen is setin the range of 1:1.0-2.0. Though air is used advantageously as the feedsource for molecular oxygen, oxygen-enriched air or pure oxygen may beused as occasion demands.

[0032] The other components of the mixed gas are nitrogen, carbondioxide, and other inert gas. The exhaust gas discharged from theabsorption column 30 may be recycled for use in the mixed gas. In thiscase, the amount of steam, the amount of molecular oxygen, and theamount of other inert gas which are necessary for the composition of themixed gas may be adjusted by the amount of the recycled gas.

[0033] In the first reaction zone, the mixed gas of the compositionmentioned above is supplied at a space velocity in the range of500-3,000 h⁻¹ (STP) per 1 m³ Of the catalyst (i) under a reactionpressure in the range of normal pressure to 0.5 MPa (gauge pressure).The reaction temperature is controlled at a level in the range of250-450° C., preferably in the range of 300-380° C.

[0034] The acrolein-containing gas which is discharged from the firstreaction zone is introduced in its unmodified form into the secondreaction zone. The acrolein-containing gas is supplied to the secondreaction zone at a space velocity in the range of 300-5,000 hr⁻¹ (STP)per 1 m³ of the catalyst (ii) under a reaction pressure in the range ofnormal pressure to 0.5 MPa (gauge pressure). The reaction temperature iscontrolled at a level in the range of 200-400° C., preferably in therange of 220-380° C. The water concentration in the mixed gas suppliedto the first reaction zone can be determined by gas chromatography, bythe Carl-Fischer method, or by the use of a hygrometer. The waterconcentration of the mixed gas may be directly measured. Otherwise, itmay be found by measuring the water concentrations of various gasesintroduced into the first reaction zone and subjecting the results ofthe measurement to calculation.

[0035] The acrylic acid-containing gas 20 which is obtained by thereaction of catalytic gas phase oxidation has a temperature in the rangeof 200-350° C. Preferably, it is cooled to a temperature in the range of100-300° C., particularly in the range of 130-270° C. before it issupplied to the absorption column 30. As the heat exchanger 23 to beused for lowering the temperature, any of the known heat exchangers maybe used. This cooling must be prevented from lowering the temperature ofthe mixture of reaction gases below the dew point of the mixture. Whenthe reaction gas has already fallen in a proper range of temperature,the cooling is not necessary as a matter of course.

[0036] In this invention, the acrylic acid-containing gas 20 is suppliedto the absorption column 30 and subjected therein to the treatment ofabsorption by the use of the absorbent 33. The acrylic acid-containinggas which is guided to the absorption column 30 generally containsacrylic acid in the range of 10-30 wt. %, acetic acid in the range of0.25 wt. %, and water in the range of 5-15 wt. %. The conditions for theabsorption including the composition of component gases for the reactiongas, the composition of the absorbent, and the temperature of absorptionmay be arbitrarily selected on the condition that they make the waterconcentration of the acrylic acid-containing solution 35 in the bottomliquid of the acrylic acid absorption column fall in the range of 1-45wt. %. The water concentration of the bottom liquid of the column ismore preferably in the range of 1-25 wt. % and particularly preferablyin the range of 1-20 wt. %. The water concentration falling short of 1wt. % is difficult to obtain and is at a disadvantage in increasing theamount of acrylic acid to be lost. Conversely, if the waterconcentration exceeds 45 wt. %, the excess will be at a disadvantage inrequiring the apparatus used at the subsequent steps such as thedistillation column to be enlarged and suffering the amount of thedehydrating liquid and the amount of the waste water to increase aswell. Further, the liquid has the highest polymerizability when the massratio of acrylic acid to water in the mixture is about 1:1 and thesolution tends to generate a polymer in the absorption column and at thesubsequent steps. Moreover, when the dehydration is effected byazeotropic distillation as illustrated in the diagram, the increase inthe amount of the water partially condensed in the dehydrating column 40induces polymerization of acrylic acid and tends to give rise to apolymer. Specifically, for the sake of dehydrating the acrylicacid-containing solution 35, the treatment of azeotropic dehydrationwhich is performed in the presence of an azeotropic solvent is generallyresorted to. If the water concentration exceeds 45 wt. %, the excesswill induce conspicuous oil-water phase separation between thehydrophobic azeotropic dehydrating solvent and water, render thedispersion of a polymerization inhibitor uneven, and dispose the acrylicacid to polymerize.

[0037] As the acrylic acid absorption column 30, any of the knowncolumns such as plate column, packed column, spray column, etc. may beused. The plate column or the packed column proves preferable where theacrylic acid concentration in the acrylic acid-containing gas to betreated is high and the polymerization in the column needs to beprevented effectively.

[0038] In the case of the packed column, for example, it is preferableto place a packing material having a relatively high efficiency ofabsorption of acrylic acid on the upstream side of the flow of theabsorbent within the column and a packing material having a relativelylow ability of forming the polymer of acrylic acid and/or plates on thedownstream side. The expression “relatively high(low)” means that when aplurality of packing materials are used, a given packing material has ahigher (lower) performance than the other packing materials. Thestatement “a packing material having a relatively low ability of forminga polymer” means that when a plurality of packing materials are usedeach in packing the acrylic acid absorption column, a given packingmaterial has a lower ability to form a polymer than the other packingmaterials. Generally, since the absorbent and the acrylicacid-containing gas are advanced into counter-current contact, the upperpart of the acrylic acid absorption column constitutes the upstream sideand the bottom side of the acrylic acid absorption column constitutesthe downstream side. In the various packing materials and plates whichare available for packing acrylic acid absorption columns, the gauzetype regular packing material ranks at the top and the sheet typeregular packing material, the irregular packing material, the grid typeregular packing material, and the plates follow it in the descendingorder in terms of the efficiency of absorption in any ordinary column.The plates which are endowed with high performance equal the sheet typeregular packing material and the irregular packing material. In terms ofthe ease with which acrylic acid polymerizes, the gauze type regularpacking material ranks at the top and the sheet type regular packingmaterial, the irregular packing material, the grid type regular packingmaterial, and the plates follow it in the descending order. When thegauze type regular packing material is used with a view to heighteningthe efficiency of absorption, for example, since the gauze type regularpacking material is liable to dispose a substance under treatment topolymerize, it poses the problem of polymerization and renders anextended operation impossible. Conversely, when the grid type regularpacking material is used, for example, with a view to preventing thepolymerization, since this packing material is deficient in theefficiency of absorption, it requires the column to have an unduly largeheight for the purpose of obtaining an expected efficiency. Thus, byusing the gauze type regular packing material on the upstream side ofthe flow of the liquid containing the solvent in the column and at leastone material selected from the group consisting of the sheet typeregular packing material, the irregular packing material, the grid typeregular packing material, and the plates, particularly preferably thesheet type regular packing material and/or the irregular packingmaterial, on the downstream side, both the ability to preventpolymerization and the efficiency of absorption are satisfied and theextended stable operation is realized. The gauze type regular packingsinclude Sulzer Packing (made by Sumitomo Heavy Industries Co., Ltd.),Technopack (made by Mitsui & Co., Ltd.), and M. C. Pack (made byMitsubishi Chemical Engineering Co., Ltd.), the sheet type regularpacking materials include MELLAPAK (made by Sumitomo Heavy IndustriesCo., Ltd.), Technopack (made by Mitsui & Co., Ltd.), and M. C. Pack(made by Mitsubishi Chemical Engineering Co., Ltd.), the grid typeregular packing materials include Flexigrid (made by KOCH EngineeringCo., Inc.), the irregular packing materials include Raschig Rings, PallRings, Cascade Mini Rings (made by Dodwell Corp.), and IMTP (made byNorton Corp.), and the plates include sieve tray, valve tray, bubble captray, buffle tray, dual flow tray, super flack tray, ripple tray and jettray. Among other irregular packing materials, Cascade Mini Rings andIMTP which permit nearly regular packing prove particularly favorablebecause they excel in the ability to prevent polymerization and exhibita high efficiency of absorption.

[0039] The top of the acrylic acid absorption column is generallyoperated under a pressure exceeding the normal pressure. The pressure inthe top of the column (gauge pressure) is properly in the range of 0-0.4MPa, preferably in the range of 0-0.1 MPa, and particularly in the rangeof 0-0.03 MPa. If this pressure is lower than 0 MPa (gauge pressure),the shortage will be at a disadvantage in necessitating a vacuum deviceand entailing a plant cost and a utility cost. Conversely, if thepressure exceeds 0.4 MPa (gauge pressure), the excess will be at adisadvantage in requiring the temperature of the absorption column to beincreased considerably with a view to expelling the low boilingsubstance from the top of the column and exalting the polymerizabilityof acrylic acid in the absorption column. The temperature of the top ofthe column is preferred to be generally in the range of 30-80° C. andparticularly in the range of 40-70° C.

[0040] This invention contemplates adjusting the water concentration ofthe acrylic acid-containing solution in the range of 1-45 wt. %. Thoughthe method for effecting this adjustment does not need to beparticularly restricted, the adjustment can be attained by varying theamount of the absorbent to be introduced. This invention prefers toeffect the absorption of acrylic acid by establishing counter-currentcontact between the acrylic acid containing gas introduced into theabsorption column and the absorbent supplied thereto at a mass flow rateof the absorbent properly in the range of 0.1-1.5 times, advantageouslyin the range of 0.2-1.3 times, and particularly advantageously in therange of 0.3-1.1 times to the mass flow rate of the propylene introducedinto the first reaction zone. If the mass flow rate of the absorbentfalls short of 0.1 times, the shortage will result in rendering theabsorption of acrylic acid difficult, decreasing the quantity of thewetting liquid in the absorption column, and inducing an extreme declinein the efficiency of the operation of the absorption column. Thequantity of the wetting liquid of the absorption column is defined bythe following formula.Quantity  of  wetting  liquid  [m³/m² ⋅ hr] = Flow  rate  of  absorbent  per  unit  time  [m³/hr]/Cross-sectional  area  of  absorption  column  [m²]

[0041] The quantity of the wetting liquid is properly not less than 0.2m³/m² hr, preferably not less than 0.8 m³/m² hr, and particularlypreferably not less than 1.0 m³/m² hr. If the mass flow rate exceeds 1.5times, the excess will results in increasing the water concentration ofthe acrylic acid-containing solution which is subjected to absorption ofacrylic acid. If the temperature of the absorption column is elevatedwith a view to fixing the water concentration in the acrylicacid-containing solution at a constant level, this elevation will be ata disadvantage in disposing the acrylic acid to polymerization.

[0042] As concrete examples of the absorbent to be supplied, such knownsolvents as water, organic acid-containing waters, and high boilinginert hydrophobic organic liquids may be cited. These absorbents may beused either singly or in the form of a mixture of two or more members.For this invention, the absorbent is preferred to have water as a maincomponent. The absorbent in the acrylic acid absorption column, forexample, possibly has a composition of 0-10 wt. % of acrylic acid, 0-20wt. % of acetic acid, and 70-100 wt. % of water.

[0043] Incidentally, this absorbent is preferred for the purpose ofpreventing such a polymerizing substance as acrylic acid frompolymerization to contain one or more compounds selected from the groupconsisting of N-oxyl compounds, phenol compounds, manganese salts suchas manganese acetate, copper dialkyldithiocarbamates such as copperdibutyl-thiocarbamate, nitroso compounds, amine compounds, andphenothiazine.

[0044] The N-oxyl compounds do not need to be particularly restrictedbut may be selected from such N-oxy compounds which are generally knownas polymerization inhibitors for vinyl compounds. Among other N-oxylcompounds, 2,2,6,6-tetramethyl piperidinoxyls represented by thefollowing formula (1):

[0045] (wherein R¹ denotes CH₂, CHOH, CHCH₂OH, CHCH₂CH₂OH, CHOCH₂OH,CHOCH₂CH₂OH, CHCOOH, or C═O and R² denotes a hydrogen atom or CH₂OH) arefavorably used. Though N-oxyl compounds are usable without beingparticularly restricted, it is proper to use one or more membersselected from among 2,2,6,6-tetramethyl piperidinoxyl,4-hydroxy-2,2,6,6-tetramethyl piperidinoxyl, and4,4′,4″-tris-(2,2,6,6-tetramethyl piperidinoxyl)phosphite which afford aproper effect of preventing polymerization. Particularly when2,26,6-tetramethyl piperidinoxyl or 4-hydroxy-2,2,6,6-tetramethylpiperidinoxyl is used as an N-oxyl compound, since it constitutes astabilizer system without requiring inclusion of a metal in the relevantcomponent, it dismisses the possibility of the metallic parts of theequipment being corroded by the stabilizer and facilitates the disposalof waste liquid as well.

[0046] As typical examples of the N-hydorxy-2,2,6,6-tetramethylpiperidine compound, 1,4-dihydroxy-2,2,6,6-tetramethyl piperidine and1-hydroxy-2,2,6,6-tetramethyl piperidine may be cited. TheseN-hydroxy-2,2,6,6-tetramethyl piperidine compounds may be used eithersingly or in the form of a mixture of two or more members.

[0047] As concrete examples of the 2,2,6,6-tetramethyl piperidinecompound, 2,2,6,6-tetramethyl piperidine and4-hydroxy-2,2,6,6-tetramethyl piperizine may be cited. These compoundsmay be used either singly or in the form of a mixture of two or moremembers. Incidentally, N-hydroxy-2,2,6,6-tetramethyl piperidinecompounds and 2,2,6,6-tetramethyl piperidine compounds are possiblycontained as impurities in commercial products of N-oxyl compounds. Theuse of such a commercially available N-oxyl compound results inadditional use of an N-hydroy-2,2,6,6-tetramethyl piperidine compoundand a 2,2,6,6-tetramethyl piperidine compound.

[0048] As concrete examples of the phenol compound, hydroquinone,methoquinone, and (p-methoxyphenol) may be cited. Methoquinone provesadvantageous because it excels hydroquinone in the effect of preventingpolymerization particularly when it is used in combination with anN-oxyyl compound or a phenothiazine compound.

[0049] As concrete examples of the phenothiazine compound,phenothiazine, bis-(α-methylbenzyl)-phenothiazine,3,7-dioctylphenothiazine, and bis-(α-dimethylbenzyl)phenothiazine may becited.

[0050] The copper salt compound does not need to be particularlyrestricted but may be selected from various inorganic and organic coppersalts. As concrete examples of the copper salt compound, copperdialkyldithiocarbamates, copper acetate, copper naphthenate, copperacrylate, copper sulfate, copper nitrate, and copper chloride may becited. The copper salt compounds of both univalent and divalent formsare usable. Among other copper salt compounds mentioned above, copperdialkyldithiocarbamates prove particularly favorable from the viewpointof effect, for example.

[0051] As concrete examples of the copper dialkyldithiocarbamate, copperdimethyldithiocarbamate, copper diethyldithiocarbamate, copperdipropyldithiocarbamate, copper dibutyldithiocarbamate, copperdipentyldithiocarbamate, copper dihexyldithiocarbamate, copperdiphenyldithiocarbamate, copper methylethyldithiocarbamate, coppermethylpropyldithiocarbamate, copper methylbutyydithiocarbamate, coppermethylpentyldithiocarbamate, copper methylhexyldithiocarbamate, coppermethylphenyldithiocarbamate, copper ethylpropyldithiocarbamate, copperethylbutyldithiocarbamate, copper ethylpentyldithiocarbamate, copperethylhexyldithiocarbamate, copper ethylphenyldithiocarbamate, copperpropylbutyldithiocarbamate, copper propylpentyldithiocarbamate, copperpropylhexyldithiocarbamate, copper propylphenyldithiocarbamate, copperbutylpentyldithiocarbamate, copper butylhexyldithiocarbamate, copperbutylphenyldithiocarbamate, copper pentylhexyldithiocarbamate, copperpentylphenyldithiocarbamate, and copper hexylphenyldithiocarbamate maybe cited. These copper dialkyldithiocarbamates may be both univalent anddivalent copper salts. Among other copper dialkyldithiocarbamatesmentioned above, copper dimethyldithiocarabamate, copperdiethyldithiocarbamate, and copper dibutyldithiocarbamate provefavorable and copper dibutyldithiocarbamate proves particularlyfavorable from the viewpoint of effect and ease of procurement.

[0052] As concrete examples of the manganese salt compound, manganesedialkyldithiocarbamates (the alkyl group may be any of methyl, ethyl,propy and butyl, and the two alkyl groups may be identical orotherwise), manganese diphenyldithiocarbamate, manganese formate,manganese acetate, manganese octanoate, manganese napthenate, manganesepermanganate, and manganese salt compounds of ethylenediaminetetraacetic acid may be cited. These manganese salt compounds may beused either singly or in the form of a mixture of two or more members.

[0053] This invention prefers the absorbent to include one or morecompounds selected from among N-oxyl compounds, phenol compounds,manganese salts, copper dialkyldithiocarbamates, nitroso compounds,amine compounds, and phenothiazines. The combined use of two or more ofthese seven kinds of compounds brings about an equal or greater effectof preventing polymerization.

[0054] The amount of the polymerization inhibitor to be used does notneed to be particularly restricted but may be properly adjusted to suitthe conditions of the relevant operation. It is, however, proper tolimit the total amount of the polymerization inhibitor to be used in therange of 3-3,500 ppm (by mass) based on the mass of the acrylic acid inthe reaction gas to be absorbed. As regards the preferred amounts of theindividual polymerization inhibitors to be used, the amount of N-oxylcompound is in the range of 1-500 ppm, the amount of manganese saltcompound or copper salt compound is in the range of 1-200 ppm, theamount of nitroso compound is in the range of 1-500 ppm, the amount ofphenol compound is in the range of 1-500 ppm, and the amount ofphenothiazine compound is in the range of 1-500 ppm respectively basedon the mass of acrylic acid in the reaction gas.

[0055] Further, the site for the supply of the polymerization inhibitorand the method for the incorporation thereof into the system do not needto be particularly restricted. It is nevertheless proper to supply thepolymerization inhibitor through the top of the acrylic acid absorptioncolumn. When the polymerization inhibitor is mixed with the absorbent inadvance of the supply thereof to the system, it is utilized effectivelybecause it is enabled to be distributed uniformly in the acrylic acidabsorption column. It is economical to reuse the absorbent which hasbeen separated in the purifying column.

[0056] This invention does not contemplate imposing any restriction onthe method for purification subsequent to the step for the absorption ofacrylic acid, because it directs adjusting the water concentration ofthe bottom liquid of the acrylic acid absorption column in a specificrange thereby improving the efficiency of absorption and stabilizing thevarious subsequent steps, and not toward restricting the purification toany specific method.

[0057] Generally, the treatment for dehydration is followed by the stepfor separating a low boiling substance, the step for separating a highboiling substance, and the other steps for purification. This inventionpermits combination of any of the methods of purification heretoforeknown to the art. The purification of acrylic acid may be effected notmerely by the method of distillation but also by proper combination ofstripping, crystallization, extraction, absorption, and partialcondensation.

[0058] The second aspect of this invention is a method for theproduction of acrylic acid comprising a step of introducing a mixed gascontaining propylene and molecular oxygen into a first reaction zonepacked with a complex oxide catalyst having molybdenum and bismuth asessential components and oxidizing propylene and obtaining anacrolein-containing gas, a step of introducing said acrolein-containinggas into a second reaction zone packed with a complex oxide catalysthaving molybdenum and vanadium as essential components and obtaining anacrylic acid-containing gas, and a step of introducing said acrylicacid-containing gas into an acrylic acid absorption column and causingit to contact an absorbent thereby obtaining an acrylic acid-containingsolution which comprises the steps of

[0059] (a) said first reaction zone and said second reaction zone beingformed by dividing reaction tubes with at least one perforated tubeplate,

[0060] (b) said propylene concentration of said mixed gas introducedinto said first reaction zone being in the range of 7-15 vol. % and thewater concentration in said mixed gas being in the range of 0-10 vol. %,and

[0061] (c) said water concentration of said acrylic acid-containingsolution obtained in the acrylic acid absorption column being adjustedto a level in the range of 1-45 wt. % by adjusting the amount of anabsorbent to be introduced.

[0062] This second invention is different from the first invention inrespect of the necessary condition of the item (c) mentioned above,namely the point that the adjustment of the water concentration of theacrylic acid-containing solution to a level in the range of 1-45 wt. %is attained by the regulation of the amount of the absorbent to beintroduced. The other necessary conditions for the second invention arethe same as those of the first invention mentioned above.

[0063] The exhaust gas 36 discharged from the top of absorption column30 contains heat, steam, unreacted propylene, acrolein, and inert gaswhich are generated by the reaction of catalytic gas phase oxidation.It, therefore, can be recycled as illustrated in FIG. 1 by being heatedwith the heat exchanger 71, then treated with the flow controller 72 tohave the flow rate thereof adjusted in the optimum range, and mixed withthe air 3 in advance of the supply thereof to the reactor 10. Thecomposition of the exhaust gas, particularly the water content thereof,however, is readily varied by the temperature of the top of theabsorption column. This invention, therefore, requires the amount of thesteam contained in the mixed gas to be limited in the range of 0-10 vol.%. The act of changing the amount of the water contained in thedischarged gas from the top of the column by varying the temperature ofthe top of the column is not only unfavorable for the sake of limitingthe amount of the steam contained in the mixed gas but also liable, whenthe temperature of the top of the column is heightened, to give rise toan acrylic acid polymer within the absorption column. In contrast, whenthe amount of the absorbent is varied, the change of the water contentof the acrylic acid-containing solution can be adjusted by varying theamount of the absorbent and the variation of the water content of thegas 36 discharged from the top of the column can be controlled.Specifically, the second invention similarly to the first invention ispreferred to effect the absorption of acrylic acid by establishingcounter-current contact between the acrylic acid including gasintroduced into the absorption column and the absorbent supplied theretoat a mass flow rate of the absorbent properly in the range of 0.1-1.5times, advantageously in the range of 0.2-1.3 times, and particularlyadvantageously in the range of 0.3-1.1 times to the mass flow rate ofthe propylene introduced into the first reaction zone.

[0064] Incidentally, the expression “by the amount of the absorbent tobe introduced” conveys a double meaning, i.e. that the waterconcentration is controlled in the range of 1-45 wt. % and that, evenwhen the water concentration remains in the range of 1-45 wt. %, thevariation thereof is further restrained within a constant range. Thesignificance of the first meaning is as already explained in the firstaspect of the invention. In contrast, the significance of the secondmeaning resides in the fact that the variation in the down stream of theprocess of production can be efficiently restrained because thevariation of the composition of the bottom liquid of the acrylic acidabsorption column constituting the upstream of the process isdiminished. In the production of acrylic acid, since numerous stepsincluding a step for separation of a low boiling substance, a step forseparation of a high boiling substance, and other steps for purificationare continuously carried out in addition to the step for absorption ofacrylic acid and the step for dehydration mentioned above, a change inthe water content of the bottom liquid of the acrylic acid absorptioncolumn, for example, results in varying the amount of the dehydratingsolvent used in the subsequent step for dehydration, the temperature andthe pressure in the interior of the dehydrating column, the amount ofthe polymerization inhibitor to be used, and the amounts of the solventphase 42 and water phase 43 to be recovered and recycled from time totime. The change further results in varying the acrylic acidconcentration of the bottom liquid of the dehydrating column and theamount of the polymerization inhibitor to be added and varying theconditions of the subsequent separation of the high boiling substance.The variations in these conditions entail the occurrence of acrylic acidpolymer at the step of purification and form a cause for degrading thequality of the product. This invention, therefore, has been directedtoward producing acrylic acid with high degree of purification byadjusting the water concentration of the acrylic acid-containingsolution in a prescribed range in the upstream of the series of stepsmentioned above thereby simplifying and conveniencing the treatments atthe subsequent downstream series of steps and restraining the variationsas well.

[0065] The purpose of enabling the water concentration of the acrylicacid-containing solution already falling in the range of 1-45 wt. % tobe further controlled in a constant range such as, for example, therange of 20+1 wt. % may be accomplished by measuring the waterconcentration of the bottom liquid of the absorption column and varyingthe amount of the absorbent to be incorporated based on the result ofthe measurement. As the methods for measuring the water concentration ofthe bottom liquid of the absorption column, a method which finds thewater concentration from the change in the electric conductivity of thebottom liquid of the column and a method which finds the waterconcentration by acidimetry are available in addition to gaschromatography and Carl Fischer process.

[0066] The acrylic acid which has been obtained by the first or thesecond invention may be supplied to a step for the production ofpolyacrylic acid (salt) to manufacture polyacrylic acid (salt) and thepolyacrylic acid thus obtained may be used for further manufacturing awater absorbing resin, for example. The third aspect of this invention,therefore, is a method for the production of polyacrylic acid comprisingusing the acrylic acid obtained by the method mentioned above.

[0067] The process for producing polyacrylic acid (salt) is enabled toproduce polyacrylic acid (salt) by introducing the acrylic acid to thestep for neutralization, the step for polymerization, the step fordrying, and the step for cooling sequentially in the order mentioned andsubjecting the acid to the relevant treatments. This process affordspolyacrylic acid when it elects to omit the neutralization of acrylicacid. For this process, therefore, the step for neutralization mentionedabove constitutes an arbitrary item. The process is allowed to perform anecessary treatment on the flow of the acrylic acid with a view toimproving varying solid state properties. It may include a cross-linkingstep during or after the step for polymerization, for example.

[0068] The step for neutralization is an arbitrary additional item forthis process. As a concrete example of this step, a method whichconsists in mixing acrylic acid or a resultant polyacrylic acid (salt)with a prescribed amount of a basic substance in the form of powder oraqueous solution may be cited. The method for this mixture does not needto be particularly restricted but may be selected from among the knownmethods. Incidentally, the step for neutralization may be carried outprior to polymerization (the neutralization with a monomer), during thecourse of polymerization, or subsequently to polymerization (theneutralization with a gel) optionally, it may be carried out both beforeand after the polymerization. In the illustrated example, the process isdepicted as carrying out the step for polymerization subsequently to thestep for neutralization. When the step for neutralization is carried outsubsequently to the step for polymerization, the construction of theapparatus may be properly modified to suit the flow of the process. Thedevice for polymerization and the device for neutralization may beidentical or not identical.

[0069] The basic substance to be used for neutralizing may be properlyselected from such known basic substances as, for example, carbonates orbicarbonates, hydrides of alkali metals, ammonia, and organic amines.The ratio of neutralization of acrylic acid does not need to beparticularly restricted. The neutralization may be adjusted so that theratio thereof will reach a level in the range of 30-100 mol %,preferably in the range of 50-80 mol %. When the heat of the reaction ofneutralization is required to be removed, this removal may be attainedby using an arbitrary cooling means, namely by introducing the productof neutralization into such a cooling device as a cooling column, forexample. When the liquid thermal medium supplied via the line 4 a isadopted as a refrigerant, it has the advantage of lowering the cost ofcooling.

[0070] The acrylic acid (salt) solution resulting from theneutralization, when necessary, is introduced into the step forpolymerization. The method for effecting the polymerization at this stepdoes not need to be particularly restricted. When the polymerization iseffected with a radical polymerization initiator, it may resort to anyof the known methods of polymerization such as radiation-inducedpolymerization, electron beam-induced polymerization, and ultravioletlight-induced polymerization using a photosensitizer. At the step forpolymerization, the acrylic acid, when necessary, may be polymerized toform an aqueous acrylic acid (salt) solution having an acrylic acidconcentration of preferably not less than 10 wt. % and more preferablynot less than 20 wt. % and preferably not more than 80 wt. % and morepreferably not more than 70 wt. %, prior to the polymerization.

[0071] In this invention, various conditions such as the kind ofpolymerization initiator and the conditions of polymerization may bearbitrarily selected. Such known additives as, for example, thecross-linking agent and other monomers and even the water-soluble chaintransfer agent and hydrophilic macromolecular compound may beincorporated, as occasion demands. For the step of polymerization, areactor or a device which is arbitrarily selected may be used. Thepolymerization device does not need to be particularly restricted butmay be selected properly from among those which are in popular use.

[0072] The polyacrylic acid (salt) which results from the polymerizationis generally a polymer in the form of a hydrogel. It is subjectedfurther to a step for drying for the purpose of expelling the watercontained therein. The method for drying this polymer does not need tobe particularly restricted. The polymer may be dried at a properlyselected temperature, preferably in the range of 70-230° C. by using anyof such known drying devices as the hot air drier, fluidized-bed drier,drum drier, and Nauter type drier. As the thermal medium to be suppliedto the step for drying, the steam which is discharged from the step forte production of acrylic acid, particularly the heat of reactionobtained from the device for catalytic gas phase oxidation, may beutilized.

[0073] The hydrogel of polyacrylic acid (salt), namely the hydratedpolymer, is thermally dried by using any of various drying devices inpopular use. The hydrogel may be dried by using such a heat conductiontype drier as the drum drier or paddle drier and exposing this hydrogelto the heating surface of the drier which has been dried with steam. Forthe sake of decreasing the residual monomer and ensuring the efficiencyof drying, the hot air conduction drying which is effected by directexposure of the hydrogen to the steam proves favorable. Specifically, bydrying the hydrogel with a hot air, namely a gas containing steam, at adew-point temperature of not lower than 50° C., preferably not lowerthan 60° C. and not higher than 90° C., preferably not higher than 80°C. and at a temperature of not lower than 100° C., preferably not lowerthan 150° C. and not higher than 200° C., preferably not higher than180° C., it is made possible to decrease the residual monomer andincrease a water absorption capacity of the polyacrylic acid (salt). Theduration of the drying may be properly selected generally in the rangeof one minute to three hours, preferably in the range of five minutes toone hour.

[0074] The polyacrylic acid (salt) which results from the step fordrying still remains at an elevated temperature when it is dischargedfrom the drying column. Properly, it is cooled at the step for coolingto a correct temperature in the range of room temperature-90° C.,preferably in the range of 40° C.-80° C. Though the method for coolingthe polyacrylic acid (salt) does not need to be particularly restricted,the drying may be attained by blowing cold wind thereon or introducingit into an arbitrary cooling device such as the refrigerator.

[0075] The polyacrylic acid (salt) which has been cooled to the expectedtemperature may be used as it is. It may further undergo granulation orpulverization to an expected shape or further incorporate thereinvarious additives such as reducing agent, spice, and binder so as tosuit the purpose for which it is finally used.

[0076] For this invention, the polyacrylic acid (salt) which has beendried is preferred to be cooled. When the hydrogel is fragmented to asize in the approximate range of one to several mm and then dried, forexample, the polyacrylic acid (salt) arising after drying is in the formof dry particles measuring approximately in the range of one to severalmm. Generally, the dry particles occurring after drying are in the formof an aggregate, depending on the kind of method employed for thedrying. When the dry polyacrylic acid (salt) is pulverized or furtherclassified, as occasion demands, to form a polyacrylic acid (salt)powder having a weight average particle diameter in the range of10-1,000 μm, preferably in the range of 100-800 μm and the powder isfurther made, when necessary, to incorporate therein various modifierssuch as, for example, the aqueous solution of a surface cross-linkingagent, granulation binder, and deodorant, it is made possible by theapplication of a cooling step not only to improve the efficiency ofpulverization and sharpen the particle size distribution but also toimprove the various solid stage properties of a water absorbing resinsuch as, for example, the water absorption capacity of the polyacrylicacid (salt) under pressure while restraining the dispersion among theindividual particles of the powder because the modifiers can beuniformly added to the powder.

EXPERIMENTS

[0077] Now, this invention will be described more specifically belowwith reference to working examples thereof.

Reference Example 1 Preparation of Catalyst

[0078] A molybdenum-bismuth type catalyst was prepared as the catalystfor use in the first reaction zone by following the procedure of Example1 of the official gazette of JP-A-2000-325795. It will be designated asCatalyst (I) herein below. A molybdenum-vanadium type catalyst wasprepared as the catalyst for use in the second reaction zone byfollowing the procedure of Example 1 of the official gazette ofJP-A-08-206504. It will be designated as Catalyst (II) herein below.Conversion  ratio  of  propylene  (%) = 100 × [(Number  of  moles  of  reacted  propylene)/(number  of  moles  of  supplied  propylene)]Yield  of  acrolein  (%) = 100 × [(Number  of  moles  of  formed  acrolein)/(number  of  moles  of  supplied  propylene)]Yield  of  acrylic  acid  (%) = 100 × [(Number  of  moles  of  formed  acrylic  acid)/(number  of  moles  of  supplied  propylene)]

Example 1

[0079] A reactor which was covered with a jacket for circulation ofthermal medium, and provided with reaction tubes measuring 25 mm ininside diameter and 7,000 mm in length therein and inserted a perforatedtube plate having a thickness of 75 mm in a position of 3,500 mm fromthe lower part of the jacket was used. The plate divides the reactorinto two vertical parts, an upper one and a lower. The resultantreaction zone (the lower part acts as a first reaction zone and theupper part acts as a second reaction zone) which allowed thetemperatures of the thermal media for the upper and the lower partsthereof to be controlled by circulating the thermal media respectivelyto the upper and the lower parts. It was packed sequentially from thelower part to the upper part of the reaction tube with (1) ceramic ballsalone having an average diameter of 5 mm, (2) a mixture formed by mixingthe catalyst (I) and ceramic balls of an average diameter of 5 mm at avolume ratio of 70:30, (3) the catalyst (I) alone, (4) Raschig ringsmade of stainless steel and measuring 5 mm in outside diameter, 4.5 mmin inside diameter, and 6 mm in length, (5) a mixture formed by mixingthe catalyst (II) and ceramic balls of an average diameter of 5 mm at avolume ratio of 75:25, and (6) the catalyst (II) alone in the ordermentioned to form layers measuring 250 mm, 700 mm, 2,300 mm, 500 mm, 600mm, and 1,900 mm respectively.

[0080] To the first reaction zone, a mixed gas comprising 8.0 vol. % ofpropylene, 14.4 vol. % of O₂, and 5.0 vol. % of H₂O (the balance formedof N₂, propane, etc) was supplied at a space velocity of 1,250 hr¹ (STP)in the first reaction zone.

[0081] At this time, the reaction was continued while the temperaturesof the thermal media to the first reaction zone and the second reactionzone were so controlled as to adjust the conversion ratio of propylenein the range of 97±0.5 mol % and the yield of acrolein in the range of1±0.5 mol % under the second reaction zone outlet pressure of 0.15 MPa(absolute pressure). After the elapse of 100 hours following the startof the reaction, the yield of acrylic acid was 87.0 mol %. The acrylicacid-containing gas obtained at this point was introduced at atemperature of 170° C. into an acrylic acid absorption column having 14steps as a theoretical number of steps to absorb the acrylic acidtherein with an absorbent water containing 1.8 wt. % of acrylic acid,5.6 wt. % of acetic acid, and such an amount of hydroquinone as equaled200 wt. ppm based on the amount of the acrylic acid in the acrylicacid-containing gas introduced into the absorption column. When theamount of the absorbent was so adjusted as to fix the waterconcentration of the bottom liquid of the absorption column at 25 wt. %under the conditions of 62.9° C. in temperature of the top of theacrylic acid absorption column and 0.11 MPa (absolute pressure) inpressure of the top of the column, an acrylic acid containing solutionhaving a target water concentration was obtained at a mass flow ratio ofthe amount of the absorbent/propylene=0.9.

[0082] The efficiency of absorption at this time was 98.3%. Theabsorption column was operated for one week without inducing any rise inthe pressure loss in the column and the neighborhood. When the columnwas then opened and inspected, the inspection did not detect a polymerwithin the column, the top, and the piping.

[0083] A sample, 5 ml in volume, of the acrylic acid containing solutionconsequently obtained was placed in a test tube. The test tube wasimmersed in an oil bath kept at 95° C. The time required for theviscosity of the sample to rise was found to be 11.5 hrs. This rise ofthe viscosity is so related with the length of the time that precedesthe start of polymerization. The short time indicates that the solutionpolymerizes easily and the long time indicates that the solutionpolymerizes only with difficulty.

Example 2

[0084] A reaction was performed by following the procedure of Example 1while changing the composition of the mixed gas supplied to the firstreaction zone to 9.0 vol. % of propylene, 16.2 vol. % of O₂, and 2.9vol. % of H₂O (the balance formed of N₂O, propane, etc.). After theelapse of 100 hours following the start of the reaction, the yield ofacrylic acid was 86.5 mol %. The acrylic acid-containing gas obtained atthis point was introduced at a temperature of 168° C. into an acrylicacid absorption column having 14 steps as a theoretical number of stepsto absorb the acrylic acid therein with an absorbent water containing1.8 wt. % of acrylic acid 6.0 wt. % of acetic acid, and such an amountof hydroquinone as equaled 200 wt. ppm based on the amount of theacrylic acid in the acrylic acid-containing gas introduced into theabsorption column. When the amount of the absorbent was so adjusted asto fix the water concentration of the bottom liquid of the absorptioncolumn at 21 wt. % under the conditions of 62.9° C. in temperature ofthe top of the acrylic acid absorption column and 0.11 MPa (absolutepressure) in pressure of the top of the column, an acrylic acidcontaining solution having a target water concentration was obtained ata mass flow ratio of the amount of the absorbent/propylene=0.8.

[0085] The efficiency of absorption at this time was 98.5%. Theabsorption column was operated for one week without inducing any rise inthe pressure loss in the column and the neighborhood. When the columnwas then opened and inspected, the inspection did not detect a polymerwithin the column, the top, and the piping.

[0086] A sample, 5 ml in volume, of the acrylic acid containing solutionconsequently obtained was placed in a test tube. The test tube wasimmersed in an oil bath kept at 95° C. The time required for theviscosity of the sample to rise was found to be 13.2 hrs.

Example 3

[0087] The same reactor as used in Example 1 was packed sequentiallyfrom the lower part to the upper part of the reaction tube with (1)ceramic balls alone having an average diameter of 5 mm, (2) a mixtureformed by mixing the catalyst (I) and ceramic balls having an averagediameter of 5 mm at a volume ratio of 55:45, (3) a mixture formed bymixing the catalyst (I) and ceramic balls having an average diameter of5 mm at a volume ratio of 70:30, (4) the catalyst (I) alone, (5) Raschigrings made of stainless steel and measuring 5 mm in outside diameter,4.5 mm in inside diameter, and 6 mm in length, (6) a mixture formed bymixing the catalyst (II) and ceramic balls having an average diameter of5 mm at a volume ratio of 65:35, (7) a mixture formed by mixing thecatalyst (II) and racemic balls having an average diameter of 5 mm at avolume ratio of 80:20, and (8) the catalyst (II) alone in the ordermentioned to form layers measuring 250 mm, 500 mm, 500 mm, 2,200 mm, 500mm, 500 mm, 500 mm, and 2,000 mm respectively.

[0088] To the first reaction zone, a mixed gas comprising 10.0 vol. % ofpropylene, 18.0 vol. % of O₂, and 0.9 vol. % of H₂O (the balance formedof N₂, propane, etc) was supplied at a space velocity of 1,250 hr⁻¹(STP) in the first reaction zone.

[0089] At this time, the reaction was continued while the temperaturesof the thermal media to the first reaction zone and the second reactionzone were so controlled as to adjust the conversion ratio of propylenein the range of 97±0.5 mol % and the yield of acrolein in the range of1±0.5 mol % under the second reaction zone outlet pressure of 0.15 MPa(absolute pressure). After the elapse of 100 hours following the startof the reaction, the yield of acrylic acid was 85.6 mol %. The acrylicacid-containing gas obtained at this point was introduced at atemperature of 167° C. into an acrylic acid absorption column having 14steps as a theoretical number of steps to absorb the acrylic acidtherein with an absorbent water containing 1.8 wt. % of acrylic acid,6.5 wt. % of acetic acid, and such an amount of hydroquinone as equaled200 wt. ppm based on the amount of the acrylic acid in the acrylicacid-containing gas introduced into the absorption column. When theamount of the absorbent was so adjusted as to fix the waterconcentration of the bottom liquid of the absorption column at 17 wt. %under the conditions of 62.9° C. in temperature of the top of theacrylic acid absorption column and 0.11 MPa (absolute pressure) inpressure of the top of the column, an acrylic acid containing solutionhaving a target water concentration was obtained at a mass flow ratio ofthe amount of the absorbent/propylene=0.7.

[0090] The efficiency of absorption at this time was 98.6%. Theabsorption column was operated for one week without inducing any rise inthe pressure loss in the column and the neighborhood. When the columnwas then opened and inspected, the inspection did not detect a polymerwithin the column, the top, and the piping.

[0091] A sample, 5 ml in volume, of the acrylic acid containing solutionconsequently obtained was placed in a test tube. The test tube wasimmersed in an oil bath kept at 95° C. The time required for theviscosity of the sample to rise was found to be 15.5 hrs.

Example 4

[0092] The acrylic acid-containing gas obtained in Example 3 wasintroduced at a temperature of 167° C. into an acrylic acid absorptioncolumn having 14 steps as theoretical number of steps to absorb acrylicacid therein with an absorbent water containing 1.8 wt. % of acrylicacid, 7.1 wt. % of acetic acid, and such an amount of hydroquinone asequaled 200 wt. ppm based on the amount of the acrylic acid in theacrylic acid-containing gas introduced into the absorption column. Whenthe amount of the absorbent water was so adjusted as to fix the waterconcentration of the bottom liquid of the absorption column at 8 wt. %under the conditions of 62.9° C. in temperature of the top of theacrylic acid absorption column and 0.11 MPa (absolute pressure) in thepressure of the top of the column, an acrylic acid containing solutionhaving a target water concentration was obtained at a mass flow ratio ofthe amount of the absorbent water/propylene=0.5.

[0093] The efficiency of absorption at this time was 97.5%. Theabsorption column was operated for one week without inducing any rise inthe pressure loss in the column and the neighborhood. When the columnwas then opened and inspected, the inspection detected practically nopolymer within the column, the top, and the piping.

[0094] A sample, 5 ml in volume, of the acrylic acid containing solutionconsequently obtained was placed in a test tube. The test tube wasimmersed in an oil bath kept at 95° C. The time required for theviscosity of the sample to rise was found to be 18.8 hrs.

Example 5

[0095] The same reactor as used in Example 1 was packed sequentiallywith the same catalysts to form layers measuring 250 mm, 800 mm, 2,200mm, 500 mm, 700 mm, and 1,800 mm respectively.

[0096] To the first reaction zone, a mixed gas comprising 8.0 vol. % ofpropylene, 14.4 vol. % of O₂, and 9.6 vol. % of H₂O (the balance formedof N₂, propane, etc) was supplied at a space velocity of 1,500 hr⁻¹(STP) in the first reaction zone.

[0097] At this time, the reaction was continued while the temperaturesof the thermal media to the first reaction zone and the second reactionzone were so controlled as to adjust the conversion ratio of propylenein the range of 97±0.5 mol % and the yield of acrolein in the range of1±0.5 mol % under the second reaction zone outlet pressure of 0.15 MPa(absolute pressure). After the elapse of 100 hours following the startof the reaction, the yield of acrylic acid was 86.3 mol %. The acrylicacid-containing gas obtained at this point was introduced at atemperature of 172° C. into an acrylic acid absorption column having 14steps as a theoretical number of steps to absorb the acrylic acidtherein with an absorbent water containing 1.8 wt. % of acrylic acid,3.8 wt. % of acetic acid, and such an amount of hydroquinone as equaled200 wt. ppm based on the amount of the acrylic acid in the acrylicacid-containing gas introduced into the absorption column. When theamount of the absorbent water was so adjusted as to fix the waterconcentration of the bottom liquid of the absorption column at 40 wt. %under the conditions of 62.9° C. in temperature of the top of theacrylic acid absorption column and 0.11 MPa (absolute pressure) inpressure of the top of the column, an acrylic acid containing solutionhaving a target water concentration was obtained at a mass flow ratio ofthe amount of the absorbent water/propylene=1.1.

[0098] The efficiency of absorption at this time was 98.9%. Theabsorption column was operated for one week without inducing any rise inthe pressure loss in the column and the neighborhood. When the columnwas then opened and inspected, the inspection did not detect a polymerwithin the column, the top, and the piping.

[0099] A sample, 5 ml in volume, of the acrylic acid containing solutionconsequently obtained was placed in a test tube. The test tube wasimmersed in an oil bath kept at 95° C. The time required for theviscosity of the sample to rise was found to be 8.2 hrs.

Example 6

[0100] A reaction was carried out by following the procedure of example4 while changing the H₂O concentration in the mixed gas supplied to thefirst reaction zone to 7.9 vol. %. After the elapse of 100 hoursfollowing the start of the reaction, the yield of acrylic acid was 86.0mol %. The acrylic acid-containing gas obtained at this point wasintroduced at a temperature of 171° C. into an acrylic acid absorptioncolumn having 14 steps as a theoretical number of steps to absorb theacrylic acid therein with an absorbent water containing 1.3 wt. % ofacrylic acid, 4.4 wt. % of acetic acid, and such an amount ofhydroquinone as equaled 200 wt. ppm based on the amount of the acrylicacid in the acrylic acid-containing gas introduced into the absorptioncolumn, with the mass flow ratio of the amount of the absorbentwater/propylene fixed at 1.6. At this time, the temperature of the topof the acrylic acid absorption column was 73.7° C., the pressure of thetop of the column was 0.11 MPa (absolute pressure), the waterconcentration of the bottom liquid of the absorption column was 25 wt.%, and the efficiency of absorption was 97.1%.

[0101] The absorption column was operated for one week without inducingany rise in the pressure loss in the column and the neighborhood. Whenthe column was then opened and inspected, the inspection detected acertain amount of polymer within the column, the top, and the piping.

[0102] A sample, 5 ml in volume, of the acrylic acid containing solutionconsequently obtained was placed in a test tube. The test tube wasimmersed in an oil bath kept at 95° C. The time required for theviscosity of the sample to rise was found to be 11.6 hrs.

Comparative Example 1

[0103] A reaction was carried out by following the procedure of Example1 while chanting the composition of the mixed gas supplied to the firstreaction zone to 6.0 vol. % of propylene, 10.8 vol. % of O₂, and 9.9vol. % of H₂O (the balance formed of N₂, propane, etc). After the elapseof 100 hours following the start of the reaction, the yield of acrylicacid was 87.8 mol %. The acrylic acid-containing gas obtained at thispoint was introduced at a temperature of 172° C. into an acrylic acidabsorption column having 14 steps as a theoretical number of steps toabsorb the acrylic acid therein with an absorbent water containing 1.8wt. % of acrylic acid, 4.8 wt. % of acetic acid, and such an amount ofhydroquinone as equaled 200 wt. ppm based on the amount of the acrylicacid in the acrylic acid-containing gas introduced into the absorptioncolumn. When the amount of the absorbent water was so adjusted as to fixthe water concentration of the bottom liquid of the absorption column at25 wt. % under the conditions of 62.9° C. in temperature of the top ofthe acrylic acid absorption column and 0.11 MPa (absolute pressure) inpressure of the top of the column, an acrylic acid containing solutionhaving a target water concentration was obtained at a mass flow ratio ofthe amount of the absorbent water/propylene=0.8.

[0104] The efficiency of absorption at this time was 95.2%. After theoperation of the absorption column had been continued for five days, itwas discontinued on account of an increase of the pressure loss in thecolumn and the neighborhood. When the column was opened and inspected,the inspection detected a considerable amount of polymer within thecolumn, the top, and the piping.

[0105] A sample, 5 ml in volume, of the acrylic acid containing solutionconsequently obtained was placed in a test tube. The test tube wasimmersed in an oil bath kept at 95° C. The time required for theviscosity of the sample to rise was found to be 11.8 hrs.

Comparative Example 2

[0106] A reaction was carried out by following the procedure of Example1 while changing the H₂O concentration in the mixed gas supplied to thefirst reaction zone to 11.6 vol. %. After the elapse of 100 hoursfollowing the start of the reaction, the yield of acrylic acid was 87.4mol %. The acrylic acid-containing gas obtained at this point wasintroduced at a temperature of 173° C. into an acrylic acid absorptioncolumn having 14 steps as a theoretical number of steps to absorb theacrylic acid therein with an absorbent water containing 1.8 wt. % ofacrylic acid, 4.1 wt. % of acetic acid, and such an amount ofhydroquinone as equaled 200 wt. ppm based on the amount of the acrylicacid in the acrylic acid-containing gas introduced into the absorptioncolumn. When the amount of the absorbent water was so adjusted as to fixthe water concentration of the bottom liquid of the absorption column at25 wt. % under the conditions of 62.9° C. in temperature of the top ofthe acrylic acid absorption column and 0.11 MPa (absolute pressure) inpressure of the top of the column, an acrylic acid containing solutionhaving a target water concentration was obtained at a mass flow ratio ofthe amount of the absorbent water/propylene=0.8.

[0107] The efficiency of absorption at this time was 94.9%. After theoperation of the absorption column had been continued for four days, itwas stopped on account of an increase in the pressure loss in the columnand the neighborhood. When the column was then opened and inspected, theinspection detected a considerable amount of polymer within the column,the top, and the piping.

[0108] A sample, 5 ml in volume, of the acrylic acid containing solutionconsequently obtained was placed in a test tube. The test tube wasimmersed in an oil bath kept at 95° C. The time required for theviscosity of the sample to rise was found to be 11.8 hrs.

Comparative Example 3

[0109] A reaction was carried out by following the procedure of Example1 while changing the H₂O concentration in the mixed gas supplied to thefirst reaction zone to 9.6 vol. %. After the elapse of 100 hoursfollowing the start of the reaction, the yield of acrylic acid was 87.1mol %. The acrylic acid-containing gas obtained at this point wasintroduced at a temperature of 171° C. into an acrylic acid absorptioncolumn having 14 steps as a theoretical number of steps to absorb theacrylic acid therein with an absorbent water containing 1.8 wt. % ofacrylic acid, 3.5 wt. % of acetic acid, and such an amount ofhydroquinone as equaled 200 wt. ppm based on the amount of the acrylicacid in the acrylic acid-containing gas introduced into the absorptioncolumn. When the amount of the absorbent water was so adjusted as to fixthe water concentration of the bottom liquid of the absorption column at47 wt. % under the conditions of 62.9° C. in temperature of the top ofthe acrylic acid absorption column and 0.11 MPa (absolute pressure) inpressure of the top of the column, an acrylic acid containing solutionhaving a target water concentration was obtained at a mass flow ratio ofthe amount of the absorbent water/propylene=1.52.

[0110] The efficiency of absorption at this time was 99.0%. Theabsorption column was operated for one week without inducing any rise inthe pressure loss in the column and the neighborhood. When the columnwas then opened and inspected, the inspection did not detect a polymerwithin the column, the top, and the piping.

[0111] A sample, 5 ml in volume, of the acrylic acid containing solutionconsequently obtained was placed in a test tube. The test tube wasimmersed in an oil bath kept at 95° C. The time required for theviscosity of the sample to rise was found to be 6.5 hrs.

1. A method for the production of acrylic acid comprising a step ofintroducing a mixed gas containing propylene and molecular oxygen into afirst reaction zone packed with a complex oxide catalyst havingmolybdenum and bismuth as essential components and oxidizing propyleneand obtaining an acrolein-containing gas, a step of introducing saidacrolein-containing gas into a second reaction zone packed with acomplex oxide catalyst having molybdenum and vanadium as essentialcomponents and obtaining an acrylic acid-containing gas, and a step ofintroducing said acrylic acid-containing gas into an acrylic acidabsorption column and causing it to contact an absorbent water therebyobtaining an acrylic acid-containing solution which comprises the stepsof (a) said first reaction zone and said second reaction zone beingformed by dividing reaction tubes with at least one perforated tubeplate, (b) said mixed gas for introduction into said first reaction zonehaving a propylene concentration in the range of 7-15 vol. % and a waterconcentration in the range of 0-10 vol. %, and (c) said acrylicacid-containing solution absorbed in said acrylic acid absorption columnhaving a water concentration in the range of 1-45 wt. %.
 2. A methodaccording to claim 1, wherein said absorbent water is introduced intosaid acrylic acid absorption column at a mass flow rate in the range of0.1-1.5 times the mass flow rate of propylene introduced into said firstreaction zone.
 3. A method according to claim 1, wherein a maincomponent of said absorbent water is water.
 4. A method for theproduction of acrylic acid comprising a step of introducing a mixed gascontaining propylene and molecular oxygen into a first reaction zonepacked with a complex oxide catalyst having molybdenum and bismuth asessential components and oxidizing propylene and obtaining anacrolein-containing gas, a step of introducing said acrolein-containinggas into a second reaction zone packed with a complex oxide catalysthaving molybdenum and vanadium as essential components and obtaining anacrylic acid-containing gas, and a step of introducing said acrylicacid-containing gas into an acrylic acid absorption column and causingit to contact an absorbent water thereby obtaining an acrylicacid-containing solution which comprises the steps of (a) said firstreaction zone and said second reaction zone being formed by dividingreaction tubes with at least one perforated tube plate, (b) saidpropylene concentration of said mixed gas introduced into said firstreaction zone being in the range of 7-15 vol. % and the waterconcentration in said mixed gas being in the range of 0-10 vol. %, and(c) said water concentration of said acrylic acid-containing solutionobtained in the acrylic acid absorption column being adjusted to a levelin the range of 1-45 wt. % by adjusting the amount of an absorbent waterto be introduced.
 5. A method according to claim 4, wherein the amountof said absorbent water to be introduced is 0.1-1.5 times the mass flowamount of propylene introduced into said first reaction zone.
 6. Amethod for the production of polyacrylic acid comprising using theacrylic acid obtained by the method set forth in any of claims 1-5.